In recent years, an improvement in fuel economy of vehicles has become an important object from the point of view of preservation of the global environment. Accordingly, the trend of strengthening materials used and reducing thickness of members to decrease vehicle body weight has been intensified. As the materials used, high strength steel sheets having a tensile strength of 540 MPa or higher are particularly required. However, since strengthening of steel sheets leads to degradation of formability, high strength steel sheets having excellent formability are desired and, particularly, there is a high demand for steel sheets having a small thickness (thin steel sheets) having excellent formability.
To satisfy the demand, various multi phase steel sheets have been proposed such as a dual phase steel sheet (DP steel sheet) having a dual phase microstructure composed of a ferrite phase and a martensite phase and a steel sheet having a multi phase microstructure including a ferrite phase, a martensite phase, and a bainite phase.
For example, in Japanese Unexamined Patent Application Publication No. 63-293121, there is a description of a method for manufacturing a high strength cold rolled steel sheet having excellent local ductility including: subjecting a cold rolled steel sheet having a composition including 0.08 to 0.30% of C, 0.1 to 2.5% of Si, 0.5 to 2.5% of Mn, and 0.01 to 0.15% of P to recrystallization annealing at a temperature equal to or higher than an Ac1 point; subsequently performing forced air-cooling to a temperature region of an Ar1 point to 600° C.; performing rapid cooling at a cooling rate equal to or higher than 100° C./s to form a multi phase microstructure composed of a ferrite phase and a low-temperature transformed phase; and subsequently performing overaging at a temperature of 350° C. to 600° C. so that a ratio Hv (L)/Hv (α) of the hardness Hv (L) of the low-temperature transformed phase to ferrite hardness Hv (α), which is obtained by a predetermined relational expression, satisfies a range of 1.5 to 3.5. In the technique described in JP 63-293121, the volume fraction of the low-temperature transformed phase is increased by increasing the quenching start temperature, and then overaging is performed at a temperature of 350° C. to 600° C. to precipitate C in the ferrite and soften the low-temperature transformed phase to thereby reduce the ratio Hv (L)/Hv (α), and thus local ductility is improved.
However, the technique described in JP 63-293121 has problems in that continuous annealing equipment is required which can perform rapid cooling (quenching) after recrystallization annealing, and it is required to add large amounts of alloying elements to suppress a rapid decrease in strength due to the overaging at a high temperature.
In addition, in Japanese Unexamined Patent Application Publication No. 05-112832, there is a description of a method for manufacturing a high strength hot rolled steel sheet with a low yield ratio having excellent corrosion resistance, including: subjecting a steel slab containing 0.02 to 0.25% of C, 2.0% or less of Si, 1.6 to 3.5% of Mn, 0.03 to 0.20% of P, 0.02% or less of S, 0.05 to 2.0% of Cu, 0.005 to 0.100% of sol.Al, and 0.008% or less of N to hot rolling to form a hot rolled coil; pickling the hot rolled coil; and subsequently annealing the hot rolled coil at a temperature of 720° C. to 950° C. by a continuous annealing line. According to the technique described in JP 05-112832, it is possible to manufacture a high strength hot rolled steel sheet which maintains a low yield ratio, high ductility, and good stretch flangeability, is excellent in corrosion resistance, and has a multi phase microstructure.
In the technique described in JP 05-112832, large amounts of P and Cu are essentially added in combination. However, when a large amount of Cu is present, hot workability is degraded; and when a large amount of P is present, steel is embrittled. In addition, P shows a strong tendency to segregate in steel, and this segregated P causes problems such as degradation of stretch flangeability of steel sheets and embrittlement of welded zones. In addition, when a large amount of P is present, wettability is degraded.
In addition, in Japanese Unexamined Patent Application Publication No. 10-60593, there is a description of a high strength cold rolled steel sheet which has a composition containing 0.03 to 0.17% of C, 1.0% or less of Si, 0.3 to 2.0% of Mn, 0.010% or less of P, 0.010% or less of S, and 0.005 to 0.06% of Al and satisfying the relation of C (%)>(3/40)×Mn and a microstructure composed of a ferrite phase and a second phase including mainly bainite or pearlite, satisfies the relation of (Vickers hardness of second phase)/(Vickers hardness of ferrite phase)<1.6, and is excellent in the balance between strength and stretch flangeability. The high strength cold rolled steel sheet described in JP 10-60593 is obtained by subjecting a steel (slab) having the above-described composition to hot rolling; coiling at a temperature equal to or lower than 650° C.; pickling; subsequently cold rolling; annealing including soaking at a temperature equal to or higher than an A1 point and equal to or lower than (A3 point+50° C.), subsequent slow cooling to a temperature T1 of 750° C. to 650° C. at a rate of 20° C./s or lower, and subsequent cooling from T1 to 500° C. at a rate of 20° C./s or higher; and subsequently overaging at a temperature of 500° C. to 250° C.
However, although the high strength cold rolled steel sheet described in JP 10-60593 has excellent stretch flangeability, in the case of a high strength of 540 MPa or higher, elongation is less than 26% and a problem occurs in that an elongation sufficient for maintaining desired excellent formability cannot be ensured.
In addition, in many cases, vehicle components are exposed to the corrosive environment and are thus required to have corrosion resistance in addition to the above-described strengthening and improvement in formability. A high strength galvanized steel sheet having excellent formability is demanded for such an application.
To satisfy the demand, for example, in Japanese Unexamined Patent Application Publication No. 04-141566, there is a description of a method for manufacturing a high strength galvannealed steel sheet including: heating a steel slab which contains 0.05 to 0.15% of C, 0.8 to 1.6% of Mn, 0.3 to 1.5% of Si, and the balance Fe with inevitable impurities and contains 0.02% or less of S as impurities, at a temperature of 1280° C. or higher; subjecting the steel slab to hot rolling with a finish temperature of 880° C. or higher to form a hot rolled sheet; annealing the hot rolled sheet in a temperature region of 750° C. to 900° C.; dipping the annealed steel sheet in a galvanizing bath in the course of cooling after the annealing; and subsequently performing alloying at a temperature of 520° C. to 640° C.
However, in JP 04-141566, it is necessary to heat a steel slab at a high temperature of 1280° C. or higher and the crystal grains become too coarse. Even after hot rolling, the microstructure of the hot rolled sheet remains coarse, and it is difficult to form a steel sheet fine microstructure after annealing. In addition, a large amount of scale loss is generated, yield is lowered, the amount of consumed energy becomes large. A problem also occurs in that the risk of generation of flaws increases. Furthermore, the target thickness is a relatively large thickness of 2.6 mm and it is unclear whether or not a high strength coated steel sheet having a small thickness and excellent formability can be manufactured according to JP 04-141566.
It could therefore be helpful to provide a high strength galvanized steel sheet that has a small sheet thickness of about 1.0 to 1.8 mm and excellent formability; and a method for manufacturing the high strength galvanized steel sheet. Here, the “high strength” means a tensile strength TS equal to or higher than 540 MPa, and preferably equal to or higher than 590 MPa. In addition, the “excellent formability” means a case in which the elongation El is equal to or greater than 30% (when using a JIS No. 5 test piece) and a hole expanding ratio λ in a hole expanding test based on the Japan Iron and Steel Federation Standard JFST 1001-1996 is equal to or higher than 80%.